1. Field of the Invention
The present invention relates to a solid electrolytic capacitor comprising a lead frame, and a method for manufacturing the same.
2. Description of the Related Art
A solid electrolytic capacitor (1) having a configuration illustrated in FIG. 5 is known (see JP H10-64761A). The solid electrolytic capacitor (1) comprises a capacitor element (2) protruding a wire-like anode lead (20). An anode lead frame (3) is attached to the anode lead (20) by resistance welding. A cathode lead frame (30) is attached to an outer surface of the capacitor element (2) using a conductive adhesive (6). A part of the capacitor element (2) and both the lead frames (3) and (30) are covered with a housing (5) made of a synthetic resin. The lead frames (3) and (30) protruding outside the housing (5) are bent downward along an outer surface of the housing (5).
In the capacitor element (2), as illustrated in FIG. 6, a dielectric oxide coating (21), a solid electrolyte layer (22) formed of a conductive polymer, and a carbon and silver layer (23) are successively formed on a surface of an anode member (24) formed of a sintered valve metal, such as tantalum or the like, and the anode lead (20) protrudes from a center portion of the anode member (24).
Both the lead frames (3) and (30) with tip portions thereof in an extended position are initially attached to the anode lead (20) and the capacitor element (2), as illustrated in FIG. 7. The tip portions of both the lead frames (3) and (30) are positioned on substantially the same plane. After base end portions of both the lead frames (3) and (30) and the capacitor element (2) are covered with the housing (5), portions protruding from the housing (5) of both the lead frames (3) and (30) are angularly bent downward. In this case, bending stress during the bending process is applied to the base end portion of the anode lead (20) (B in FIG. 7). Although the base end portion is covered with the housing (5), the housing (5) is made of a synthetic resin and therefore is soft, so that the housing (5) has a small effect of relaxing force applied to the base end portion of the anode lead (20).
Therefore, when the bending stress is applied to the base end portion of the anode lead (20), the dielectric oxide coating (21) is damaged in the vicinity of the base end portion, likely leading to an increase in leakage current of the capacitor (1).
In addition, the solid electrolytic capacitor (1) of this type is often used in high frequency circuits, and there is a demand for a reduction in the Equivalent Series Resistance (ESR) of the solid electrolytic capacitor (1) so as to decrease the impedance of the whole circuit. It is considered that the ESR is reduced by increasing the diameter of the anode lead (20) to decrease the whole resistance of the anode lead (20).
However, if the diameter of the anode lead (20) is increased while the tip portions of both the lead frames (3) and (30) are positioned on substantially the same plane as illustrated in FIG. 7, a problem arises as illustrated in FIG. 8. Specifically, both the lead frames (3) and (30) are attached while the capacitor element (2) is tilted in an amount corresponding to an increase in the diameter of the anode lead (20). In this case, contact areas between the lead frames (3) and (30) and outer surfaces of the anode lead (20) and the capacitor element (2) are reduced, resulting in an increase in the ESR. If the capacitor element (2) is covered with the housing (5) in this situation, an excessively large load is applied to the capacitor element (2), leading to an increase in leakage current.
In view of this point, a configuration illustrated in FIG. 9 has been proposed (see Japanese Patent No. 3157722). In this configuration, an anode lead frame (3) is bent in a housing (5) to provide a step portion (4) which is lowered by a step. The step portion (4) includes a first horizontal portion (40), and a second horizontal portion (41) which is provided at an end of the first horizontal portion (40) and is positioned lower than the first horizontal portion (40). An anode lead (20) is welded onto a lower surface of the second horizontal portion (41). A part of bending stress of the anode lead frame (3) is received by the step portion (4), so that the bending stress applied to a base end portion of the anode lead (20) is relaxed. Thereby, the possibility that the leakage current of the capacitor (1) increases is reduced.
If the height of the second horizontal portion (41) is changed, depending on the diameter of the anode lead (20), the capacitor element (2) can be attached in an appropriate attitude even when the anode lead (20) is thick.
However, the conventional configuration has the following drawbacks.
Since the anode lead (20) is welded onto the lower surface of the second horizontal portion (41) which is angularly bent from the first horizontal portion (40), the contact area between the anode lead (20) and the anode lead frame (3) is reduced as compared to the configuration of FIG. 5. Particularly, as illustrated in an enlarged view in FIG. 10, an arc surface is formed at a portion where the step portion (4) is angularly bent as indicated with arrow C, and the anode lead (20) is not welded onto the arc surface, so that the contact area between the anode lead (20) and the anode lead frame (3) is further reduced, leading to an increase in the ESR. Also, the weld strength between the anode lead (20) and the anode lead frame (3) is reduced.